As tissue oxygen tension is an important variable in cancer therapy, it would beof major clinical benefit to be able to measure this parameter non-invasively. Current methods for determining pO2 in clinical settings are limited to sup erficial tumors. In this study, we measured the apparent diffusion constant (ADC) in an implanted murine fibrosarcoma (RIF-1) using magnetic resonance imaging and correlated the ADC with tissue pO2 measured by electron paramagnetic resonance oximetry. The ADC correlates significantly with tissue pO2 in this tumor (r=0.89; n=14) and so may be useful for providing a non-invasive index of pO2 in tumors.